1. Field of the Invention
The present invention relates to free radical initiated polymerization of polymerizable ethylenically unsaturated monomers, and more particularly, to polymerization in the presence of hydrogen peroxide
2. Brief Description of the Prior Art
Free radical initiated polymerization of polymerizable ethylenically unsaturated monomers in organic medium is usually accomplished with a so-called oil-soluble free radical initiator such as an organic peroxide or an azo compound which is soluble in the organic medium. Hydrogen peroxide is a well-known free radical initiator for addition polymerizations of ethylenically unsaturated monomers. However, its use is primarily for polymerization in aqueous emulsion polymerization techniques and it is not as well known for use in non-aqueous polymerization which, as mentioned above, uses oil-soluble free radical initiators. However, these materials are relatively expensive and in the case of certain of the azo compounds will not be available because of health and safety problems.
The use of aqueous hydrogen peroxide as a free radical initiator in non-aqueous polymerization is, however, known in the art. U.S. Pat. Nos. 3,370,050 and 3,366,605 disclose the use of aqueous hydrogen peroxide as a free radical initiator in addition polymerization to form interpolymers of hydroxyalkyl esters of unsaturated acids and to form interpolymers of ethylenically unsaturated amides. In accordance with these patents, the aqueous hydrogen peroxide is added along with the polymerizable monomers to a reaction zone and the reaction mixture is heated to reflux to conduct the polymerization. The water added with the peroxide is removed during the latter stages or after polymerization has been completed. Although using aqueous hydrogen peroxide in this fashion is reported as being effective in preparing the interpolymers described in U.S. Pat. Nos. 3,366,605 and 3,370,050, these interpolymers are of relatively high molecular weight, and it has been found that using aqueous hydrogen peroxide in this fashion is not particularly effective in preparing low molecular weight acrylic polymers having a molecular weight of 4000 or less. These polymers are becoming of increasing interest in the coatings industry where, because of their low molecular weight and resultantly low viscosities, they can be formulated into coating compositions with little organic solvent thereby complying with environmental regulations dealing with solvent contents in coating compositions. It has been found that conducting the reaction at reflux in the presence of water limits the temperature of reaction that is achievable, and high reaction temperatures have been found to be preferred for preparing low molecular weight polymers. Also, the presence of an aqueous free radical initiator in the reaction medium results in heterophase polymerization, that is, polymerization in both water and organic phases which results in the preparation of a polymer with a relatively broad molecular weight distribution leading to undesirably high viscosities.